SBCT Final EIS - Govsupport.us

Chapter 3 ⎯ Affected Environmentuifer system of the north hydrologic sector. The eastern side of KTA is in the northern end of theKoolauloa aquifer system of the windward hydrologic sector.Groundwater in the Kawailoa aquifer system is thought to drain northwest toward the Waimea coast.Since 1927, annual groundwater pumping from the Kawailoa aquifer system reportedly remained below9 mgd and ranged from 1.5 mgd in 1936 to 8.9 mgd in 1970. The State of Hawaii estimates thesustainable yield of the Kawailoa aquifer system at 39 mgd. Regional groundwater flow in theKoolauloa aquifer system is believed to be to the north or the northeast in this part of KTA. The Stateof Hawaii estimates the sustainable yield of the aquifer system at 35 mgd (USAG-HI 2004).Most of KTA is within the Northwest Rift Zone of the Koolau Volcano. The Northwest Rift Zone isdensely intruded by volcanic dikes, and the groundwater system at higher elevations is dominated bydike-impounded groundwater. The Northwest Rift Zone is primarily within the Koolauloa aquifersystem but extends into the Kawailoa aquifer system. The extreme northwest side of KTA marks thewestern boundary of the Northwest Rift Zone, where dike density decreases.The coastal plain north and east of KTA is underlain by sedimentary deposits, including alluvial depositsand limestone caprock. Drum Road crosses the upper portions of the Kawailoa and Waialuaaquifer systems in the north hydrologic unit and the central part of the Wahiawa aquifer system in thecentral hydrologic unit.Groundwater QualityGroundwater in the high-level groundwater system of the Koolau Mountain Range is generally ofvery good quality and is used as a drinking water source. On the coastal plain, groundwater has beenaffected by agricultural contamination. Groundwater in the Koolau Loa aquifer system has been affectedby pesticides used in sugar cultivation, including dibromochloropropane (DBCP) and 1,2,3-trichloropropane (HDOH 1999). Groundwater beneath the coastal plain north of KTA has been affectedby nitrates and sulfates associated with crop fertilizers and irrigation (Tenorio et al. 1970).3.1.2.4 Pohakuloa Training AreaSurface Water OccurrenceOn the Island of Hawaii, there are few defined watersheds because the young, highly permeable rockand soil deposits generally absorb the precipitation without forming stream channels. The exception isalong the island’s northern coast, where streams are better defined.The climate at PTA is classified as cool and tropical. The average annual precipitation at PTA rangesfrom 10 to 16 inches. PTA lies within the Northwest Mauna Loa and the West Mauna Kea watersheds,which drain to the northern Hualalai and southern Kohala coasts, respectively (Mink and Lau1993). The Keamuku Parcel and the PTA Trail are mainly within the West Mauna Kea watershed.The two watersheds are underlain by aquifer “sectors” of the same name.There are no surface streams, lakes, or other bodies of water within PTA boundaries due to low rainfall,porous soils, and lava substrates. Rainfall, fog drip, and occasional frost are the main sources ofwater that sustain plants and animals in the dryland habitat of PTA and Keamuku Parcel. There are noperennial streams within 15 miles of PTA. However, there are at least seven intermittent streams thatdrain surface water off the southwestern flank of Mauna Kea and lie within the same drainage area asthe PTA. Popo’s Gulch is the closest stream to PTA boundaries. There are three intermittent streamslocated within 2 miles of the cantonment area (Waikahalulu Gulch, Pohakuloa Gulch, and an unnamedgulch) which collect runoff from the southern flank of Mauna Kea (USACE 1997).February 2008 3–26 2/25th SBCT Final EIS